This invention relates to an internal combustion engine and particularly a four-cycle internal combustion engine and a control valve arrangement for controlling the variable valve timing mechanism of such engines.
It has been acknowledged that the performance of four cycle engines can be substantially improved during a wider range of engine running conditions if a variable valve timing mechanism (VVT) is employed. By changing the valve timing, it is possible to obtain the optimum valve timing conditions for each engine running condition.
Some variable valve timing mechanisms adjust the timing of only one of the camshafts. However, considerable improvement can be obtained in engine performance if both camshafts have a variable valve timing mechanisms associated with them. Although this can significantly improve the engine performance, it also makes the overall engine construction more complicated.
Basically, most VVT mechanisms include a drive to the camshaft from the engine crankshaft that includes a mechanism that is operative to change the phase angle between the driven camshaft and the crankshaft. In this way, it is possible to vary the variable valve timing during engine running. Frequently, these mechanisms are hydraulically operated and often use the same hydraulic fluid that is utilized to lubricate the engine.
In order to control the valve timing and specifically the operation of the VVT mechanism, a control valve selectively pressurizes or depressurizes one or more chambers of the variable valve timing mechanism. This requires the provision of not only a control valve and an actuator for it but also the provision of supply passages for delivering the lubricant to and from the control valve to the variable valve timing mechanism. Where two adjacent camshafts are controlled, the structure generally is duplicated and hence, the problems of locating passages, et cetera and the valves and the valve controls become more complicated.
Normally, it has been the practice to position the control valves on the engine side of the drive for the variable valve timing mechanism which is generally provided in a timing case or cover at one end of the engine. Hence, the valves are disposed at some axial distance from the variable valve timing mechanisms with which they cooperate. This gives rise to certain problems in connection with not only engine size but also in the provision of adequate passages for supplying and transferring the actuating fluid between the components.
It is, therefore, a principal object of this invention to provide an improved and compact variable valve timing control mechanism for an engine having a pair of adjacent camshafts, each of which has a variable valve timing mechanism associated with it.
It is a further object to this invention to provide an improved engine construction and control valve arrangement for a VVT mechanism that can be compact and shorten the supply passages and passages interconnecting the valves to the VVT mechanisms.
Another problem in connection with the supply of lubricating oil to the variable valve timing mechanisms is that of pressure drop. Obviously, a fairly significant pressure is required in order to achieve the shifting of the variable valve timing mechanism, particularly when this shifting occurs when the valves are being opened and closed. Therefore, any pressure drops or pressure variations can cause difficulties in operation.
Generally it has been the practice to provide a separate supply passage that extends from a source to each of the control valves. As noted previously, these control valves are also located rather remotely from the variable valve timing mechanism so pressure variations obviously become a problem.
It is, therefore, a still further object to this invention to provide an improved control arrangement for an engine VVT mechanism that is compact and which minimizes the number and length of the necessary fluid passages.